Urea grease

ABSTRACT

A urea grease of the invention is prepared by applying shear to a mixture of a monoamine compound and a diisocyanate compound to cause a reaction in the mixture, in which the monoamine compound consists of an alicyclic monoamine, and the urea grease has Peak High32-64s of 10 or less and Level High32-64s of 40 or less according to an FAG method.

TECHNICAL FIELD

The present invention relates to a urea grease.

BACKGROUND ART

A typical manufacturing method of a urea grease includes: mixing a base oil with isocyanate to prepare a first solution and heat the first solution to about 60 degrees C.; mixing a base oil with amine to prepare a second solution and heat the second solution to about 60 degrees C.; adding the second solution to the first solution with stirring and keeping stirring; heating the obtained mixture to about 160 degrees C.; and subsequently cooling the mixture to the room temperature. However, this method takes time for manufacturing (i.e., synthetic reaction) and is likely to generate micelle particles (so-called lumps) formed of a thickener. Large lumps are known for deteriorating an acoustic property when the grease is used in a slide device such as a bearing. It is also speculated that such large lumps hamper a smooth motion of the slide device such as a bearing to generate vibration and the like, thereby decreasing a motion accuracy of the device. Further, since the thickener having a non-uniform structure formed of large lumps less contributes to an inherent performance of the grease, an efficiency of the thickener is decreased. In other words, a large amount of the thickener is occasionally required for obtaining a predetermined hardness and oil separation degree.

Accordingly, a grease manufacturing method for inhibiting formation of large lumps and improving an acoustic property has been proposed (see Patent Literatures 1 and 2). The manufacturing method disclosed in Patent Literature 1 includes: a method of feeding an amine solution (or an isocyanate solution) with liquid drops having a diameter of 300 μm or less into an isocyanate solution (or an amine solution) using a spray nozzle; and a method of spraying the above solutions to each other for reaction. This manufacturing method restricts a particle diameter of each of lumps formed of the thickener (a urea compound) to less than 100 μm (about several tens μm). The manufacturing method disclosed in Patent Literature 2 includes a method of applying pressure to an amine solution and an isocyanate solution using a pressure device to increase the pressure to a predetermined pressure, and mixing the solutions by colliding with each other for reaction. This manufacturing method restricts a size of each of lumps to a range from about several tens μm to several hundreds μm.

CITATION LIST Patent Literature(s)

Patent Literature 1: JP-A-2000-248290

Patent Literature 2: JP-A-3-190996

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The urea greases obtained by the typical manufacturing methods disclosed in Patent Literatures 1 and 2 tend to have large lumps. In the manufacturing method disclosed in Patent Literature 1, there is a possibility to generate lumps having the maximum size of 300 μm or more. In the manufacturing method disclosed in Patent Literature 2, there is a possibility that large lumps each having a size ranging from about several tens μm to several hundreds μm are present. Moreover, in general, a grease formed only of cyclohexylamine exhibits an insufficient acoustic property.

Accordingly, an object of the invention is to provide a urea grease having an excellent acoustic property with a small number of lumps.

Means for Solving the Problems

In order to solve the above problem, the invention provides the following urea grease. (1) A urea grease prepared by applying shear to a mixture of a monoamine compound and a diisocyanate compound to cause a reaction in the mixture, in which the monoamine compound consists of an alicyclic monoamine, and the urea grease has Peak High32-64s of 10 or less and Level High32-64s of 40 or less according to an FAG method.

According to the above aspect of the invention, a urea grease having an excellent acoustic property with a small number of lumps can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an example of a manufacturing device of a urea grease in an exemplary embodiment of the invention.

FIG. 2 schematically shows a lateral side and a top of the manufacturing device in FIG. 1,

FIG. 3 schematically shows a lateral side and a top of a manufacturing device of a urea grease in another exemplary embodiment of the invention.

FIG. 4 is an optical micrograph of a urea grease manufactured in Example 1 of the invention.

FIG. 5 is an optical micrograph of a urea grease manufactured in Example 2 of the invention.

FIG. 6 is an optical micrograph of a urea grease manufactured in Example 3 of the invention.

FIG. 7 is an optical micrograph of a urea grease manufactured in Comparative 1 of the invention.

DESCRIPTION OF EMBODIMENT(S)

A urea grease in an exemplary embodiment of the invention (hereinafter, also referred to as “the present grease”) is obtained using a thickener obtained by reacting a monoamine compound and a diisocyanate compound in a solution. The monoamine compound is provided only in a form of cyclohexyl amine. The urea grease has Peak High32-64s of 10 or less and Level High32-64s of 40 or less according to an FAG method. The exemplary embodiment of the invention will be described below in detail.

Constitution of Grease

The base oil used in the present grease is not particularly limited, but may be any mineral base oil and synthetic base oil typically usable for manufacturing a grease. One of the mineral base oil and synthetic base oil may be alone or a mixture thereof may be used.

Usable mineral oils are obtained by purification in an appropriate combination of vacuum distillation, solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, sulfate cleaning, clay purification hydrorefining and the like. Examples of the synthetic base oil include polyalphaolefin (PAO) base oil, other hydrocarbon base oil, ester base oil, alkyldiphenylether base oil, polyalkylene glycol base oil (PAG), and alkylbenzene base oil.

The thickener used in the present grease is obtained by reacting the monoamine compound and the diisocyanate compound in the solution. In the exemplary embodiment, the monoamine compound needs to be only in a form of an alicyclic monoamine. Examples of the alicyclic monoamine include cyclohexyl amine, alkylcyclohexyl amine, cyclobutyl airline, cyclopentyl amine, and cycloheptyl amine. One of the alicyclic monoamines may be used alone or a plurality of alicyclic monoamines may be mixed in use.

Examples of the diisocyanate compound include diphenylmethane-4,4′-diisocyanate (MDI), tolylene diisocyanate, and naphthylene-1,5-diisocyanate. One of the di isocyanates may be used alone or a plurality of diisocyanates may be mixed in use.

The present grease has Peak High3-64s of 10 or less and Level High32-64s of 40 or less according to the FAG method.

A required level of each of the Peak High32-64s and the Level High32-64s depends on usage. However, the Peak High32-64s exceeding 10 is insufficient since the acoustic property is in the same level as that of a conventional art. The Peak High32-64s is preferably 7 or less, more preferably 3 or less.

Moreover, the Level High32-64s exceeding 40 is insufficient since the acoustic property is in the same level as that of a conventional art. The Level High32-64s is preferably 30 or less, more preferably 15 or less.

Herein, the Peak High32-64s and Level High32-64s according to the FAG method can be measured using an acoustic measurement device dedicated for a grease (“Grease Test Rig Be Quiet+” manufactured by SKr). Specifically, a bearing dedicated for an acoustic measurement, in which a grease is not put, is set in the acoustic measurement device. While the bearing is being rotated at a predetermined speed, acoustic data is obtained after the elapse from 32 seconds to 64 seconds since the bearing starts rotating. Further, a predetermined amount of a sample (grease) is put in the bearing. While the bearing is being rotated at a predetermined speed, acoustic data is obtained after the elapse from 32 seconds to 64 seconds since the bearing starts rotating. The acoustic data is analyzed by a program stored in the acoustic measurement device, so that a Peak High value and a Level High value are obtained. An analysis result of each of the Peak High value and the Level High value is usually obtained as an average value of n=3 to 5.

A method for providing the Peak High32-64s and Level High32-64s obtainable by the FAG method in the above-described range is exemplified by a later-described manufacturing method of the present grease under a uniform high shear.

Various additives may be further added to the present grease. Examples of the additives include an antioxidant, extreme pressure agent, and rust inhibitor.

Examples of the antioxidant include: an amine antioxidant such as alkylated diphenylamine, phenyl-α-naphthylamine and alkylated-α-naphthylamine; and a phenol antioxidant such as 2,6-di-t-butyl-4-methylphenol and 4,4-methylenebis(2,6-di-t-butylphenol). A content of the antioxidant is preferably in a range from approximately 0.05 mass % to 5 mass % based on a total amount of the grease.

Examples of the extreme pressure agent are thiocarbamates such as zinc dialkyldithiophosphate, molybdenum dialkyldithiophosphate, as dithiocarbamate, zinc dithiocarbamate and molybdenum dithiocarbamate, sulfur compound (sulfurized fat and oil, sulfurized olefin, polysulfide, sulfurized mineral oil, thiophosphates, thioterpenes and dialkylthiodipropionates), phosphates and phosphites (tricresyl phosphate and triphenyl phosphite). A content of the extreme pressure agent is preferably in a range from approximately 0.1 mass % to 5 mass % based on the total amount of the grease.

Examples of the rust inhibitor include benzotriazole, zinc stearate, succinate, succinic acid derivative, thiadiazole, benzotriazole, benzotriazole derivative, sodium nitrite, petroleum sulphonate, sorbitan monooleate, fatty acid soap and amine compound. A content of the rust inhibitor is preferably in a range from approximately 0.01 mass % to 10 mass % based on the total amount of the grease.

One of the above various additives may be blended alone, or alternatively, a plurality of those may be blended in combination.

Manufacturing Method of Grease

The present grease can be manufactured, for instance, by a later-described manufacturing method of the present grease (hereinafter, also referred to as “the present manufacturing method”). In the present manufacturing method, a first base oil containing the monoamine compound and a second base oil containing the diisocyanate compound are mixed to prepare a mixture and a shear rate of 10³ s⁻¹ or more is applied to the mixture. In other words, within a short time after the first base oil and the second base are mixed, high shear is applied to the mixture. Subsequently, the monoamine compound and the diisocyanate compound are mixed and dispersed to react with each other, thereby preparing a thickener. The present manufacturing method will be described below in detail.

Base Oil

The first base oil and the second base oil usable in the present manufacturing method are not particularly limited, but may be any base oils usable in the present grease.

A kinematic viscosity at 40 degrees C. of each of the first base oil and the second base oil is preferably in a range from 10 m²/s to 600 mm²/s.

Considering compatibility of the first base oil and the second base oil, the first base oil and the second base oil preferably have similar polar characteristics and similar viscosity characteristics. Accordingly, the first base oil and the second base oil are most preferably the same base oil in use.

Thickener

In the present manufacturing method, the thickener is formed from the monoamine compound and the diisocyanate compound.

As the monoamine compound and the diisocyanate compound, the examples of those usable in the present grease are usable.

The diisocyanate compound and the monoamine compound are continuously introduced at a molar ratio of 1:2 into a reactor (a grease manufacturing device) and are immediately subjected to a high shear as described later to be mixed and reacted with each other, so that a diurea grease having less large lumps can be manufactured. Moreover, the above-described mixture of the diisocyanate compound and the monoamine compound is continuously introduced at equivalent amounts of an isocyanate group and an amino group into a reactor (a grease manufacturing device) and are similarly subjected to a high shear to be mixed and reacted with each other, so that a urea grease having less large lumps can be manufactured.

Manufacturing Method of Grease

In the present manufacturing method, the first base oil containing the monoamine compound and the second base oil containing the diisocyanate compound are mixed to prepare the mixture and a minimum shear rate of 10² s⁻¹ or more is applied to the mixture. In other words, in order to inhibit formation or growth of the lumps, it is crucial to apply a high shear to the mixture within the shortest time as possible after the first base oil and the second base oil are put into the reactor.

Specifically, a time elapsed before applying the above shear rate after putting the first base oil and the second base oil in the reactor is preferably within 15 minutes, more preferably within 5 minutes, further preferably within 10 seconds. Since a reaction starts after the diisocyanate compound and the monoamine compound are well mixed and dispersed, when the elapsed time is shorter, molecules of the thickener are less likely to form a thick bundle and a large lump.

The minimum shear rate applied to the above mixture is 10² s⁻¹ or more as described above, preferably 10³ s⁻¹ or more. A higher shear rate provides a more improved dispersion condition of the diisocyanate compound and the monoamine compound and the formed thickener, thereby providing a more uniformed grease structure. In other words, the molecules of the thickener do not form a thick bundle and a large lump.

Considering safety of the device and heat generated by shear and the like and removal of the heat, the minimum shear rate applied to the above mixture is preferably 10⁷ s⁻¹ or less.

The above shear rate can be applied to the mixture, for instance, by introducing the mixture into a reactor configured to cause shear by relative movement of facing wall surfaces.

A grease manufacturing device (the reactor) capable of generating such a high shear rate is exemplified by a manufacturing device structured as shown in FIG. 1. FIG. 2 schematically shows a lateral side and a top of the manufacturing device in FIG. 1.

The manufacturing device shown in FIG. 1 is configured to mix two types of base oils and uniformly apply high shear to the obtained mixture within an extremely short time. The high shear is applied to the mixture by a gap (a, b) between a high-speed rotating portion and an inner wall of the reactor. A diameter of the high-speed rotating portion may be constant (a=b) in a direction of a rotary shaft, or alternatively, the gap may be different. The gap may be adjusted by changing the diameter of the high-speed rotating portion in the direction of the rotary shaft, or alternatively, by providing the high-speed rotating portion in a form of a truncated cone and vertically moving the high-speed rotating portion with respect to an inner wall of a tapered reactor.

Further, the portions having a large gap may be provided by a screw or a spiral having continuous inclination, whereby extrusion capability may be provided to the high-speed rotating portion.

FIG. 3 shows a reactor (a manufacturing device of a grease) having a structure different from that of the reactor in FIG. 1, the portions having different gaps are disposed in a rotation direction. In this manufacturing device, the portions having a large gap may be inclined relative to a rotary shaft, whereby extrusion capability as provided by a screw may be provided to the high-speed rotating portion.

In the above reactor, a ratio (Max/Min) of a maximum shear rate (Max) to a minimum shear rate (Min) is preferably 100 or less, more preferably 70 or less, further preferably 50 or less, particularly preferably 10 or less. When the shear rate applied to the mixture is as uniform as possible, a grease having a uniform structure without having grown lumps is provided.

Herein, the maximum shear rate (Max) refers to a maximum shear rate applied to the mixture and the minimum shear rate (Min) refers to a minimum shear rate applied to the mixture. The maximum shear rate (Max) and the minimum shear rate (Min) are defined as follows, for instance, in the reactor shown in FIG. 1.

Max=(a linear velocity of a surface of the high-speed rotating portion at the minimum gap between the surface of the high-speed rotating portion and an inner wall surface of the reactor/the gap)

Min=(a linear velocity of a surface of the high-speed rotating portion at the maximum gap between the surface of the high-speed rotating portion and the inner wall surface of the reactor/the gap)

In FIG. 1, the gap used for calculating Max is a and the gap used for calculating Min is b.

Since a smaller Max/Min is preferable as described above, ideally a=b. In other words, in case of the reactor as shown in FIG. 1, the high-speed rotating portion is most preferably a cylinder vertically having a uniform diameter.

When the manufacturing device manufactures a urea grease, the manufacturing device may have a structure as shown in FIG. 3.

The present manufacturing method is applicable to all grease manufacturing methods including mixing a solution of the first base oil and the monoamine compound with a solution of the second base oil and the diisocyanate compound. Although a temperature condition for manufacturing the thickener differs depending on the precursors to be used, the temperature in a range from approximately 50 degrees C. to 200 degrees C. is preferable when manufacturing urea as the thickener. When the temperature is equal to or more than 50 degrees C. or more, isocyanate is likely to be solved in the base oil. When the temperature is equal to or less than 200 degrees C., deterioration of the base oil can be sufficiently inhibited. A temperature of a solution of the base oil and amine before being introduced into the reactor is preferably in a range from approximately 50 degrees C. to 100 degrees C.

In the present manufacturing method, the grease obtained by the above manufacturing method may be further kneaded. For this kneading, a roll mill generally used for manufacturing a grease is usable. The above grease may be subjected to the roll mill twice or more.

In the present manufacturing method, the grease obtained by the above manufacturing method may be further heated to the temperature in a range from 80 degrees C. to 200 degrees C. Further, for uniform heating, the grease may be kneaded and stirred. A furnace and the like may be used for heating.

EXAMPLES

The invention will be described in further detail with reference to Examples and Comparatives, but the description is mere illustrative but not exhaustive of the invention. Specifically, a urea grease was manufactured under the following various conditions and properties of the obtained grease were evaluated.

Example 1

A grease was manufactured using a urea grease manufacturing device as shown in FIG. 3. A grease manufacturing method was specifically performed as follows.

A PAO base oil (poly-α-olefin (a kinematic viscosity at 40 degrees C. of 63 mm²/s, a kinematic viscosity at 100 degrees C. of 9.8 mm²/s) containing cyclohexylamine of 28.1 mass %) heated at 70 degrees C. and a PAO base oil (poly-α-olefin (a kinematic viscosity at 40 degrees C. of 63 mm²/s, a kinematic viscosity at 100 degrees C. of 9.8 m²/s) containing MDI of 11.0 mass %) also heated at 70 degrees C. were continuously introduced at respective flow rates of 144 mL/min and 504 mL/min into a manufacturing device. Immediately after the introduction, a maximum shear rate of 42,000 s⁻¹ was applied to the obtained mixture by a high-speed rotating portion when the mixture passes a gap. A ratio (Max/Min of the maximum shear rate (Max) to the minimum shear rate (Min) when the mixture passes the gap was 1.03. A time elapsed before applying the maximum shear rate to the mixture after mixing the above two base oils was about three seconds. An amount of the thickener in the manufactured grease was 15 mass % based on a total amount of the grease.

A lump formation state of the obtained grease was observed with an optical microscope. The same applies to later-described greases in Examples and Comparatives.

Example 2

A grease was manufactured in the same manner as in Example 1, except that the maximum shear rate applied to the mixture was 83,900s⁻¹. An amount of the thickener in the manufactured grease was 15 mass % based on the total amount of the grease.

Example 3

A grease was manufactured in the same manner as in Example 1, except that a concentration of cyclohexylamine was 23.8 mass %, a concentration of MDI was 9.0 mass %, the flow rate of the amine solution was 100 mL/min, the flow rate of the MDI solution was 330 mL/min, and the maximum shear rate was 216,000 s⁻¹. An amount of the thickener in the manufactured grease was 12 mass % based on the total amount of the grease.

Comparative 1

A urea grease was manufactured by a typical method. Specifically, a PAO base oil (poly-α-olefin (a kinematic viscosity at 40 degrees C. of 63 mm²/s, a kinematic viscosity at 100 degrees C. of 9.8 mm²/s) containing cyclohexylamine of 15.7 mass %) kept at 60 degrees C. was dropped into a PAO base oil (poly-α-olefin (a kinematic viscosity at 40 degrees C. of 63 mm²/s, kinematic viscosity at 100 degrees C. of 9.8 mm²/s) containing MDI of 14.5 mass %) kept at 60 degrees C. while being stirred by an impeller. After the amine solution was dropped therein, the mixture was heated to 160 degrees C. and maintained for one hour while being kept stirred (stirring speed: 250 rpm). Subsequently, the mixture was left to be cooled while being stirred. An amount of the thickener in the manufactured grease was 15 mass % based on the total amount of the grease. The maximum shear rate during the manufacturing was about 100 s⁻¹.

Evaluation of Grease

The grease was evaluated by the following method in terms of Peak High32-64s, Level High3-64s, a mean diameter of the lumps, a number density of the lumps, and a worked penetration. The obtained results are shown in Table 1. The maximum shear rate, the minimum shear rate, and the ratio (Max/Min) of the maximum shear rate (Max) to the minimum shear rate (Min) during manufacturing of each of the greases are also shown in Table 1. Further, FIGS. 4 to 7 show optical micrographs of the greases, (1) Peak High32-64s and Level High32-64s

Peak High32-64s and Level High32-64s are measured using an acoustic measurement device dedicated for a grease (“Grease Test Rig Be Quiet+” manufactured by SKF). Specifically, a bearing dedicated for an acoustic measurement, in which a grease is not put, is set in the acoustic measurement device. While the bearing is being rotated at a predetermined speed, acoustic data is obtained after the elapse from 32 seconds to 64 seconds since the bearing starts rotating. Further, a predetermined amount of a sample (grease) is put in the bearing. While the bearing is being rotated at a predetermined speed, acoustic data is obtained after the elapse from 32 seconds to 64 seconds since the bearing starts rotating. The acoustic data is analyzed by a program stored in the acoustic measurement device, so that a Peak High value and a Level High value are obtained. An analysis result of each of the Peak High value and the Level High value is usually obtained as an average value of n=3 to 5. (2) Mean Diameter of Lumps with Diameter of 15 μm or more and Number Density of Lumps with Diameter of 15 μm or more A base oil of 5 g was put into a grease of 10 g and mixed with a spoon. Subsequently, the obtained mixture was subjected to a roll mill to be homogenized. An extremely small amount of the homogenized mixture was laid on a glass slide, covered with Saran Wrap (registered trademark) (thickness: 11 μm) as a spacer, covered with a cover glass, and further covered with another glass slide. A vertical load of about 20 N was evenly applied on the thus covered mixture to crush the grease into a film. As the roll mill, a three roll mill, model 50 (roll diameter=50 mm) manufactured by EXAKT Technologies, Inc., was used. The glass slide on the top was removed. Four or more of optical micrographs of the grease were randomly taken at 100-fold magnification by an optical microscope (OLYMPUS CORPORATION, BH-2).

At this time, it was noted to randomly take the optical micrographs so as not to intentionally select a part where a small number or a large number of lumps were found. Since the greases in Examples had small lumps with an unclear outline, a difference in a refractive index between the lumps and parts other than the lumps was increased by diluting each of the greases with the base oil.

Moreover, in fear of an unclear outline in the optical micrograph taken at a high magnification, the optical micrographs were taken at 100-fold magnification.

These micrographs were visually checked. In the micrographs except for a micrograph having the minimum number of lumps and a micrograph having the maximum amount of lumps, a size and a number of the lumps were measured using an image-editing software (VFIX-H3M (Ver. 1.0) manufactured by KEYENCE CORPORATION). A distance between two points (i.e., two-point distance) of a longer diameter of each of the lumps was manually measured using a mouse. At this time, all the lumps having a longer diameter of at least 10 μm were measured. Six points each with a measurement area of 215 μm×215 μm (a frame size of 8 cm×8 cm) were measured per micrograph. In order to improve a statistical accuracy, a larger number (e.g., at least five) of micrographs of a sample having a small number of lumps need to be used for the measurement.

Lumps having the longer diameter of 15 μm or more were selected based on the obtained two-point distances (i.e., the longer diameter of each of the lumps). A mean value of the selected lumps having the longer diameter of 15 μm or more was calculated as a mean diameter thereof. Lumps having the longer diameter of 15 μm or less were excluded because of a large measurement error and occasional difficulty in accurately determining a lump on an image.

A number density of the lumps having the longer diameter of 15 μm or more was calculated according to the following equation. A number density of the lumps having the longer diameter of 15 μm or more=(number of the lumps having the longer diameter of 15 μm or more/observation area)×dilution factor using the base oil. Herein, when the base oil of 5 g is added to the grease of 10 g, the dilution factor using the base oil is 1.5. (3) Worked Penetration

A worked penetration was measured by a method in accordance with the description of JIS K2220.

TABLE 1 Ratio of Maximum Number Mean Shear Rate to Density of Diameter of Maximum Minimum Minimum Shear Lumps of 15 Lumps of 15 Shear Rate Shear Rate Rate (Max/Min) Peak High Level High μm or more μm or more Worked (s⁻¹) (s⁻¹) (—) 32-64 s 32-64 s (pieces/mm²) (μm) Penetration Example 1 42000 40800 1.03 6.17 28.35 217 19.7 277 Example 2 83900 81500 1.03 2.78 13.94 223 21.6 231 Example 3 216000 210000 1.03 1.66 11.50 87 19.1 254 Comparative 1 about 100 1.23 81 20.60 90.23 315 27.2 245

It has been confirmed from the results shown in Table 1 that all of the urea greases (Examples 1 to 3) of the invention exhibit excellent acoustic property and uniformity with lumps small in size and number. Particularly, in Example 3 employing an extremely large shear rate of 216,000 s⁻¹, an excellent acoustic property with a small number of lumps has been achieved.

In contrast, the urea grease manufactured in Comparative 1 by a typical method exhibits an insufficient acoustic property and an extremely poor uniformity with large mean diameter and number density of the lumps.

It is also apparent from the optical micrographs shown in FIGS. 4 to 7 that the lumps in Examples are smaller and more homogenized than the lumps in Comparative.

Example 4

A grease was manufactured using a urea grease manufacturing device as shown in FIG. 1. A grease manufacturing method was specifically performed as follows.

A 500N mineral oil (having a kinematic viscosity at 40 degrees C. of 90 mm/s and containing MDI of 6.76 mass %) heated at 70 degrees C. and a 500N mineral oil (having a kinematic viscosity at 40 degrees C. 90 mm²/s and containing cyclohexylamine of 10.3 mass heated at 70 degrees C. were continuously introduced at respect flow rates of 325 mL/min and 175 mL/min into a manufacturing device. Immediately after the introduction, a maximum shear rate of 216,000 s⁻¹ was applied to the obtained mixture by a high-speed rotating portion when the mixture passes a gap. The minimum shear rate (Min) when the mixture passes the gap was 210,000 s⁻¹. The ratio Max/Min) of the maximum shear rate (Max) to the minimum shear rate (Min) when the mixture passes the gap was 1.03. A time elapsed before applying the maximum shear rate to the mixture after mixing the above two base oils was about three seconds. The grease discharged from the manufacturing device was put into a container preheated at 60 degrees C. While being stirred at 250 rpm, the grease was immediately heated up to 120 degrees C., maintained for 30 minutes and further heated up to 160 degrees C. for one hour. Subsequently, the grease was left to be cooled while being kept stirred, and subjected to a roll mill twice to obtain a grease. An amount of the thickener in the obtained grease was 8 mass % based on the total amount of the grease.

Evaluation of Grease

The grease was evaluated by the above method in terms of Peak High32-64s, Level High32-64s, and a worked penetration. The obtained results are shown in Table 2. The amount of the thickener in the each of the greases as well as the maximum shear rate, the minimum shear rate, and the ratio (Max/Min) of the maximum shear rate (Max) to the minimum shear rate (Min) during manufacturing of each of the greases are also shown in Table 2.

TABLE 2 Ratio of Maximum Maximum Minimum Shear Rate to Shear Rate Shear Rate Minimum Shear Max Min Rate Max/Min Peak High Level High Worked (s⁻¹) (s⁻¹) (—) 32-64 s 32-64 s Penetration Example 4 216,000 210,000 1.03 1.87 7.48 314

According to Table 2, in Example 4, a urea grease having an excellent acoustic property was obtained. 

1: A urea grease, prepared by a method comprising: applying shear to a mixture of a monoamine compound and a diisocyanate compound to cause a reaction in the mixture, wherein the monoamine compound consists of an alicyclic monoamine, and the urea grease has Peak High32-64s of 10 or less and Level High32-64s of 40 or less according to an FAG method. 2: The urea grease according to claim 1, wherein the alicyclic monoamine consists of cyclohexylamine. 3: The urea grease according to claim 1, wherein the shear is applied at a shear rate of 10² s⁻¹ or more. 4: The urea grease according to claim 1, wherein the shear is applied at a shear rate of 10³ s⁻¹ or more. 5: The urea grease according to claim 1, wherein the mixture is prepared by mixing a first base oil comprising the monoamine compound and a second base oil comprising the diisocyanate compound. 6: The urea grease according to claim 5, wherein the shear is applied to the mixture within 15 minutes after mixing the first base oil and the second base oil. 7: The urea grease according to claim 1, wherein the shear is applied at a shear rate of 10⁷s⁻¹ or less. 8: The urea grease according to claim 1, wherein ratio of a maximum shear rate to a minimum shear rate in the shear applied to the mixture is 70 or less. 